The present invention relates to lubricant compositions for automatic transmissions. More particularly, for automatic transmissions equipped with a mechanism to control automobile slip, characterized by high transmission torque capacity, and other favorable properties, such as those related to anti-shudder property, anti-shudder durability and prevention of clogging of materials.
A lubricant for an automatic transmission can be used in an automobile equipped with a gear and hydraulic mechanism, wet type clutch, and so on. This type of lubricant required to have a variety of properties, because it functions as the power transmission medium for the torque converter, and hydraulic and control systems, as the lubricant and heating medium for the gears, bearings and wet type clutch, and also as the lubricant medium and the medium to maintain the friction-related characteristics of the friction material. It must perform these functions well for the smooth operation of the automatic transmission.
Recently, lock-up clutches have been built in torque converters in many automobiles to improve mileage. A lock-up clutch is a device that directly transmits the engine driving force to the transmission under varying running conditions. Torque converter efficiency can be enhanced when switching between torque converter driving and direct driving is well timed.
However, a conventional lock-up mechanism works only in a high-speed range and not in a low speed range. This results in power transmission loss (revolutions per minute) between engine output and transmission input, decreasing gas mileage, during the starting period and in other low speed conditions. In an attempt to decrease that power transmission loss, some automobiles have a lock-up mechanism that works at a low automatic transmission speed. In such a case, slip control is adopted to help the lock-up clutch work in the low speed range. However, abnormal vibration of the car body, known as shudder, has been frequently observed at the lock-up clutch's friction surface, when the lock-up mechanism is operated at low speed. Such a phenomenon is more pronounced, when coefficient of friction decreases as relative sliding velocity increases at the slip-controlled lock-up clutch. In order to prevent the shudder phenomenon, the lubricant is required to have good .mu. (coefficient of friction)-V (sliding velocity) characteristics. In other words, it is required to have a coefficient of friction which increases as sliding velocity increases.
Esters of phosphates, aliphatic acids and fatty amides have been proposed as friction modifiers for automatic transmission lubricants, as disclosed by Japanese Laid-open Patent application 63-254196. However, these modifiers have disadvantages which result in a decreasing coefficient of friction at the lock-up clutch, and in insufficient transmission torque.
The applicants of the present invention have proposed the use of metallic salts of alkyl phenates and sulfided alkyl phenates (Japanese Patent Laid-open 5-105892), and to simultaneously use an organic metallic salt, such as calcium sulfonates, and a specific polyamide compound (Japanese Laid-open Patent application 8-319494). However, the friction material still clogs even in the presence of the above modifiers, when the lubricant is used for an extended period, which causes degradation of the friction-related properties, such as lowered coefficient of friction of the lock-up clutch and degraded anti-shudder property. Therefore, demands are increasing for an automatic transmission lubricant, which has a high transmission torque capacity, improved anti-shudder property, extended anti-shudder and durability for preventing clogging of the friction material, and development of the related techniques has been in strong demand.
It is an object of the present invention to provide automatic transmission lubricant compositions, which exhibit high anti-shudder durability when the lock-up mechanism is in service at low speed and for automatic transmissions equipped with a slip-controlled mechanism. The lubricant compositions have high durability and work for extended periods without being degraded, are excellent for preventing clogging of material, and provide sufficient transmission torque capacity.